President Barack Obama wipes his brow while unveiling plans in June 2013 to reduce carbon pollution. Sen. Ron Johnson said he cast earlier votes to block Obama administration attempts to pass regulations "that Congress had earlier refused to enact.”

On Aug. 14, 2013 -- more than three years before U.S. Sen. Ron Johnson could face re-election -- a national environmental group hit him with an attack ad.

The first-term Wisconsin Republican responded by using the TV spot to solicit political contributions.

We didn’t think we’d have to go into campaign mode yet for the 2016 elections. But we’re game.

In the ad, the League of Conservation Voters alleges that Johnson "has taken more than $100,000 from oil and gas and voted to let them keep spewing unlimited carbon pollution into our air."

Johnson has received $109,550 from oil and gas interests since his campaign for the 2010 election, according to the nonpartisan Center for Responsive Politics, which tracks campaign contributions. So that is not really in dispute.

Let’s see if he voted in favor of unlimited carbon pollution.

Getting our bearings

To be clear on terminology, we'll start with four points.

1. Global warming:The world’s leading science academies have long warned that the planet is warming. The pace has slowed in the past 15 years, although 2000 through 2009 was the warmest decade on record, according to NASA. In August 2013 the Intergovernmental Panel on Climate Change, an international panel of scientists, reiterated the role of human beings, saying in a draft report the odds are at least 95 percent that humans are the main cause of the warming in recent decades.

2. Greenhouse gases:The main human-caused producer of global warming is the emission of greenhouse gases, which trap the Earth's heat.

3. Carbon dioxide: Carbon dioxide makes up 84 percent of greenhouse gas emissions in the United States, according to the U.S. Environmental Protection Agency, which has determined that greenhouse gases are a threat to humans.

4. Carbon pollution:Carbon dioxide -- or carbon pollution, the term used in the ad -- comes largely from burning fossil fuels such as coal, oil and natural gas.

Like Johnson, many congressional Republicans elected in 2010 have expressed doubt about the science of global warming and opposed regulation of climate-altering gases.

The ad’s evidence

The Washington, D.C.-based League of Conservation Voters is "a liberal-leaning organization that raises money primarily in support of environmentally friendly public policy," according to the Center for Responsive Politics.

In the 2011-2012 election cycle, the five candidates who received the most money from the league -- Wisconsin U.S. Sen. Tammy Baldwin among them -- all were Democrats.

To back its claim against Johnson, the League of Conservation Voters cited three Senate votes, each an amendment to a bill; one was in 2011 and two were on the same day in 2013. Two of the votes were noted in the ad and the third was cited by the league as additional evidence.

1. EPA regulations, April 6, 2011

Johnson voted for a failed amendment to an unrelated small-business bill. The amendment would have prohibited the EPA from regulating greenhouse gas emissions and repealed the agency’s scientific finding that carbon dioxide and other heat-trapping gases are endangering human health and the environment.

So, under Johnson’s vote, the EPA couldn’t regulate any carbon emissions, including those emitted in the extraction and refining of oil, or by gasoline-burning vehicles.

2. Global warming, March 22, 2013

Johnson voted for a failed amendment to the 2014 federal budget bill. The amendment would have prohibited "further greenhouse gas regulations for the purposes of addressing climate change," which includes global warming.

Like the earlier vote, Johnson supported the status quo: no federal limits on emissions of carbon dioxide, or carbon pollution, whether by oil and gas companies or other energy producers.

3. Carbon emissions, March 22, 2013

Johnson voted for another amendment to the budget to requirea vote of 60 senators in order to enact a new federal tax or fee on carbon emissions.The amendment passed.

This measure doesn’t regulate carbon emissions, per se, but does make it more difficult to tax emissions. Such a tax, arguably, could lead to less production of energy that results in carbon emissions.

For our purposes, the third vote really isn’t necessary in evaluating the claim against Johnson. The intent of the other two votes is clear.

She also called the ad misleading because it showed smokestacks that suggested the production of electricity, and said gas and oil companies are not responsible for the carbon emissions used to make electricity.

But the question here is the effect of the measures Johnson voted for.

Chip Knappenberger, assistant director of the Center for the Study of Science at the Cato Institute, a libertarian think tank, told us that as a practical matter, the only way to reduce carbon pollution is to reduce the amount of fossil fuel produced. But he said Johnson's votes do preserve the status quo, which has no limits on greenhouse gas emissions.

Our rating

The League of Conservation Voters said Johnson voted to let oil and gas companies "keep spewing unlimited carbon pollution into our air."

Johnson voted twice to prohibit the EPA from regulating any carbon emissions.

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